Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation is converted into electrical energy. The earliest fuel cells were first constructed by William Grove in 1829 with later development efforts resuming in the late 1930's with the work of F. T. Bacon. In early experiments, hydrogen and oxygen gas were bubbled into compartments containing water that were connected by a barrier through which an aqueous electrolyte was permitted to pass. When composite graphite/platinum electrodes were submerged into each compartment and the electrodes were conductively coupled, a complete circuit was formed and redox reactions took place in the cell: hydrogen gas was oxidized to form protons at the anode (e.g., “hydrogen electrode”) and electrons were liberated to flow to the cathode (e.g., “oxygen electrode”) where they subsequently combined with oxygen.
Since that time, interest in the development of viable commercial and consumer-level fuel cell technology has been renewed. In addition to various other benefits compared with conventional methods, fuel cells generally promise improved power production with higher energy densities. For example, a typical hydrogen-oxygen cell operating at about 250° C. and a pressure of about 50 atmospheres yields approximately 1 volt of electric potential with the generation of water and a small quantity of thermal energy as byproducts. More recently, however, modern Polymer Electrolyte Membrane Fuel Cells (PEMFC's) operating at much lower temperatures and pressures (i.e., on the order of about 80° C. and about 1.3 atmospheres) have been observed to produce nearly the same voltage potential.
An additional advantage of fuel cells is that they generally have a higher energy density and are intrinsically more efficient than methods involving indirect energy conversion. In fact, fuel cell efficiencies have been typically measured at nearly twice those of thermoelectric conversion methods (i.e., fossil fuel combustion heat exchange).
With respect to portable power supply applications, fuel cells function under different principles as compared with standard batteries. As a standard battery operates, various chemical components of the electrodes are depleted over time. In a fuel cell, however, as long as fuel and oxidant are continuously supplied, the cell's electrode material is not consumed and therefore will not run down or require recharging or replacement.
One class of fuel cells currently under development for general consumer use are hydrogen fuel cells, wherein hydrogen-rich compounds (i.e., methanol) are used to fuel the redox reaction. As chemical fuel species are oxidized at the anode, electrons are liberated to flow through the external circuit. The remaining positively-charged ions (i.e., protons) then move through the electrolyte toward the cathode where they are subsequently reduced. The free electrons combine with, for example, protons and oxygen to produce water—an environmentally clean byproduct.
Current interest in perfluorinated ionomers, such as, for example, NAFION® (a perfluorinated polymer available from DuPont Microcircuit Materials, E. I. Du Pont de Nemours and Company, 14 T. W. Alexander Drive, Research Triangle Park, N.C., USA), stems from their potential use as polymer electrolyte membranes in fuel cell applications. See, for example, R. Lemons, J. Power Sources 29, 251 (1990). NAFION is a phase separated material with a crystalline region consisting of a hydrophobic TEFLON® (also available from DuPont Microcircuit Materials, E. I. Du Pont de Nemours and Company, 14 T. W. Alexander Drive, Research Triangle Park, N.C., USA) backbone and a hydrophilic ionic domain comprising randomly attached long pendant chains terminating with sulfonic acid groups. The terminal acid functionality is generally analogous to that of trifluoromethane sulfonic acid (e.g., triflic acid).
NAFION belongs to a class of polymers referred to as ion-containing polymers (e.g., ‘jonomers’). Although extensive work has been undertaken to characterize ionomers, the state and structure of ion aggregation and the resulting modifications occurring upon hydration have not been well understood, even though the importance of such considerations has generally been appreciated. See, for example, A. Eisenberg, H. L. Yeager (Eds.), Perfluorinated Ionomer Membranes, ACS Symp. Ser. 180, American Chemical Society, Washington, D.C. (1982).
Early small-angle x-ray scattering and thermo-rheological studies suggest that the ions in NAFION are clustered, containing some fluoro-carbon material. See, for example, S. C. Yeo, A. Eisenberg, J. Appl. Polymer Sci. 21, 1875 (1977). Ion clustering was further supported by both wide- and small-angle diffraction studies on hydrolyzed NAFION. See, for example, T. D. Gierke et al., J. Polymer Sci. Polymer Physics, Ed. 19, 1687 (1981); W. Y. Hsu, T. D. Gierke, Macromolecules 15, 101 (1982); and W. Y. Hsu, T. D. Gierke, J. Memb. Sci. 13, 307 (1983). This work provided a microstructure model where the structure of the system was proposed as consisting of an inverted micelle with —SO3 groups forming hydrated clusters embedded in the fluorocarbon phase with diameters from 40 to 50 Å. It was further concluded from infrared studies of water (H2O, D2O and HDO) in NAFION that the hydrated ion clusters were either much smaller than earlier estimated or were highly non-spherical in shape with frequent local intrusions of the fluorocarbon phase. See, for example, M. Falk, Can. J. Chem. 58, 1495 (1980). This work seemed to indicate that a substantial proportion of water molecules were exposed to the fluorocarbon environment. More recently, however, through reexamination of the data, a lamellar morphology was proposed for NAFION that, upon hydrolysis, creates polar sulfonic acid domains of relatively large surface area parallel to one another and connected by tie molecules. See, for example, M. H. Litt, Polymer Preprints 38, 80 (1997). A similar micelle structure was experimentally observed for NAFION solubilized in DMF. See, for example, A. V. Rebrov et al., Polymer Science U.S.S.R. 32, 251 (1990).
As NAFION membranes may function both as separators and electrolytes in fuel cell applications, the overall performance of the fuel cell is strongly influenced by the conductivity of the membrane, which itself is a function of the state of hydration of the membrane. See, for example, T. A. Zawodzinski et al., J. Electro-Chem. Soc. 95, 6040 (1991); T. E. Springer et al., J. Electro-Chem. Soc. 138, 2334 (1991); T. A. Zawodzinski et al., J. Electro-Chem. Soc. 140, 1981 (1993); and T. A. Zawodzinski et al., Solid State Ionics 60, 1993 (1993). The water content of the membrane may largely be determined inter alia by the interplay of at least three processes: (a) water absorption by the membrane; (b) transport of water through the hydrated membrane by means of, for example, the protonic current (e.g., electro-osmotic drag); and (c) water diffusion effected by means of, for example, water activity gradients. Experimental measurements of electro-osmotic drag for various sulfonated membranes over a wide range of water content have suggested that wall effects tend to dominate proton transport and that the mechanism responsible involves the tethering of sulfonic acid groups bound to water. Until recently, however, no molecular-level understanding was available for electro-osmosis. See, for example, S. J. Paddison, T. A. Zawodzinski, Solid State Ionics 113–115, 333–340 (1998).
In Direct Methanol Fuel Cells (DMFC's), aqueous methanol (CH3OH) is introduced at the anode where the fuel is electrochemically oxidized to produce CO2, protons and electrons. With conventional catalysts (typically carbon supported platinum or platinum alloys) and under the current operating temperature limitations (on the order of about 110° C.) not all of the methanol is oxidized. Due to the miscible nature of water and methanol, along with the permeability of the Polymer Electrolyte Membrane (PEM), the latter is adsorbed by the membrane causing inter alia increased swelling (essentially due to an increase in the water concentration) of the membrane. During operation of the fuel cell, protonic current within the membrane drags water (e.g., ‘electro-osmotic drag’) from the anode to the cathode, which reduces the efficiency of the fuel cell by hindering the reduction reaction at the cathode (e.g., ‘cathode flooding’). This, in turn, generally requires the utilization of relatively expensive water management techniques typically involving capture or return of water to the anode side of the DMFC. The electro-osmotic drag coefficient (e.g., the number of water molecules dragged per proton) typically increases substantially upon swelling of the membrane. Accordingly, despite the efforts of the prior art, one problem warranting resolution is the characterization of the increased electro-osmotic drag of water by protons in membranes used in the manufacture of PEMFC's. Accordingly, in one representative and exemplary aspect, the present invention proposes a molecular-based non-equilibrium statistical mechanical method for predicting diffusion coefficients for sulfonic acid based PEM's. Moreover, a representative novel ionomeric PEM material is described for preventing or otherwise ameliorating electro-osmotic drag.